1. Technical Field of the Invention
The present invention relates generally to optical fibers having an active cladding and more particularly to such optical fibers wherein the active sources located in the cladding are polarized to increase the core injection efficiency of the fiber.
2. Discussion of the Related Art
Absorption and emission of evanescent waves are well-known phenomena that have been theoretically and experimentally investigated and widely used for sensing purposes. For example, absorption of evanescent waves is used to determine the concentration of methane-gas with a tapered optical fiber. In this approach, a He--Ne laser excites bound modes in the fiber. The chemical species surrounding the tapered region of the fiber absorbs the evanescent wave associated with these modes at a specific wavelength. This absorption can be detected at the end of the fiber as a decrease in the output signal level and the concentration of the species inferred.
Using evanescent wave coupling, an optical fiber sensor has been developed with a fluorescent cladding to detect molecular oxygen. Evanescent waves are a factor whenever radiation is totally internally reflected between two dielectric media having different indices of refraction. Although most of the incident power is reflected, part of the radiation, termed the evanescent component of the field, penetrates a very thin layer of the dielectric having the lower index of refraction. Specifically, an optical fiber is clad during manufacture with a polymer such as polydimethyl siloxane which has a fluorescent dye dissolved therein. The dye itself is sensitive to the presence of molecular oxygen. The fluorescent cladding was excited via evanescent waves upon side-illumination at a wavelength within the excitation range of the dye. As before, some light was trapped in the core by evanescent coupling.
The active sources in the cladding thus produce light waves. In most cases, the active sources are usually treated as many infinitesimal electric dipole currents having random phase and random orientation which excite radiation fields and bound modes in the optical fiber. The assumption of random orientation is justified in many cases since that is the orientation usually found by fluorescent molecules. An optical fiber source for chemical detection having an active cladding is disclosed in U.S. Pat. No. 4,834,496 to Blyler, Jr. et al., which issued May 30, 1989, the specification of which is hereby incorporated by reference. It is desirable to increase the power efficient P.sub.eff (defined below) of an optical fiber having an active cladding.